In the field of dyeing keratinic fibers, hair and furs, the metaphenylenediamines play an important role which has been known for a long time. They form part of the class of compounds currently called "couplers". The couplers, in association with the paraphenylenediamines or the paraaminophenols--compounds called oxidation bases--produce in an alkaline oxidizing medium colored indamines, indoanilines or indophenols.
The association of the metaphenylenediamines with the paraphenylenediamines in an oxidizing alkaline atmosphere, and in particular in the presence of hydrogen peroxide, produce indamines capable of imparting to keratinic fibers very strong blue colors.
Moreover, the metaphenylenediamines associated with the paraaminophenols produce in an oxidizing alkaline atmosphere, indoanilines, which impart to keratinic fibers more or less purple-red colorations.
The metaphenylenediamines thus play, when considered as couplers, a double role in capillary dyeing: Contribution of blue and red, that is to say contribution of two fundamental colors which are indispensable to obtain not only blacks and greys, but also burnt or copper chestnuts.
Despite its important role this category of couplers is presently restricted in practice to a very limited number of compounds. This very limited number is explained by the fact that one may only keep for capillary dyeing on the one hand those beneficial compounds of a good innocuousness, and on the other hand the compounds permitting to obtain dyes of good qualities, that is to say, which do not change with time, in particular neither under light, nor inclement weather. By way of example, metatoluylene diamine is not satisfactory for reasons of non-innocuousness, while 1-methyl-2-amino-N-methyl-4-amino benzene, 2-amino-N-methyl-4-amino anisole, 2-amino-4-amino-N-methyl anisole, 2-amino-N-.beta.-hydroxyethyl 4-amino anisole, 1-methyl 2-amino-N-.beta.-hydroxyethyl-4-amino benzene lead to shades which are unstable with time, both in darkness and in the light. This instability is due principally to the ready cyclization into azinic molecules of the indoamines or indoanilines formed in situ during the oxidation dyeing, the azines formed then undergoing rapid photochemical degradation in the light.